Conventional particle analyzers using optical-type flow cytometers are well known. In this type of particle analyzer, a sample containing particles such as blood or the like flows through the flow cell and light from a light source is irradiated on the flow channel. The light from the particles is detected, and the type and number of particles are determined based on the detected light.
When the flow cell is irradiated by the light from the light source, the light is scattered by the particles flowing through the flow cell. Among all the scattered light, the light which scatters in the forward direction (forward scattered light) is focused on the light receiving surface of the light detecting unit via the light focusing lens. The forward scattered light is used to detect the feature quantities, that is, size, morphological information and the like, of the particles.
The direct light transmitted from the light source through the flow cell must be eliminated in order for the light detecting unit to detect the forward scattered light. Therefore, a beam stopper for blocking direct light is arranged within the light receiving optical system. Japanese Laid-Open Patent Application No. H02-304332 discloses a beam stopper arranged in the optical path between a light focusing lens and a light detecting unit.
When the beam stopper is deployed in this way, part of the forward scattered light, in addition to the direct light, is eliminated by the beam stopper. Hence, there is a need to reduce as much as possible the amount of forward scattered light eliminated by the beam stopper.